It is a great skill to have. I do not think it is anything to be alarmed about. One logical explanation can be that if her mother worked during her pregnancy, her profession as a speech pathologist could have had a good influence on the baby inside her. I am no expert but this could be one possibility. Or of course, there could be genetic factors - a field which we have yet to figure out completely. And of course, their sign language helping from day 1, could have had an impact too. If someone can forward this to the parents, hopefully they have one or two new ideas (if not already) about this whole thing. God has created each person as unique or special. This ability would definitely help her ahead in future, where in today's competitive world, she can maybe read faster than others or something. Continue the good work.